1. Field of the Invention
The invention relates to sensing, monitoring and alerting functions. In particular, it employs novel electronic circuits to detect, monitor and alert to a present condition based on the location of the interface between materials having different coefficients of refraction, such as may occur typically at the interface between water in a stream and sediments thereunder during a scour event.
2. Description of the Prior Art
Scour is a severe problem that results in millions of dollars of damage to infrastructure and substantial loss of life annually. Scour occurs during times of high tides, hurricanes, rapid river flow, and icing conditions, when sediment, including rocks, gravel, sand, and silt, are transported by currents, undermining bridge and pier foundations, submarine utility cables, and pipelines, and filling in navigational channels. Scour is dynamic; ablation and deposition can occur during the same high-energy hydrodynamic event. The net effect of scour has not been easily predicted, nor readily monitored, in real-time heretofore.
Bridge scour monitoring technologies are known. In U.S. Pat. No. 5,784,338, issued Jul. 21, 1998 to Norbert E. Yankielun et al, an instrument called a xe2x80x9ctime domain reflectomerxe2x80x9d (TDR) is directly connected to a parallel transmission line consisting of a pair of robust, specially fabricated non-corroding rods or wires (hereinafter xe2x80x9cleadsxe2x80x9d). The principle of TDR is generally known, described in the technical literature, and applied to numerous measurements and testing applications. The technique was applied to scour detection and monitoring in the aforesaid ""338 patent, which is incorporated herein by reference. TDR operates by generating an electromagnetic pulse, or a fast rise time step, and coupling it to a transmission line. The pulse travels down the transmission line at a fixed and calculable velocity, a function of the speed of light and the electrical and physical characteristics of the transmission line. The pulse propagates down the transmission line until the end of the line is reached, and is then reflected back toward the source. The time in seconds that it takes for the pulse to propagate down and back the length of the transmission line is called the xe2x80x9cround trip travel timexe2x80x9d and is calculated as described in the ""338 patent. For a two-wire parallel transmission line, changes in the dielectric media in the immediate surrounding volume cause a change in the roundtrip travel time of a pulse initiated thereon. Further, at any boundary between differing media located along the transmission line (e.g., air/water, water/sediment, etc.) a discontinuity exists that is characterized by a change in the refractive index from one medium to the next. As a pulse imposed on the transmission line encounters these boundaries, a portion of the pulse is reflected back to its source. The remaining portion of the pulse continues on to encounter other boundaries with like results, or the end of the transmission line from which it is reflected, in whole or part, back to its source. Measuring the time of flight of the reflected pulse(s), while knowing the refractive index of the media through which it passes enables one to determine where along the transmission line these boundaries are
Freshwater has a relatively high dielectric constant and dry sedimentary materials (e.g.: soil, gravel and stone) have a relatively low dielectric constant. Wet sediment has a dielectric constant that is a mixture of the constants of water and dry soil. The dielectric constant of this mixture will vary, depending upon the local sedimentary material constituency. However, in all cases of bulk dielectric, the bulk index of refraction of the mixture will be less than that of liquid water alone and significantly greater than that of the dry sedimentary materials. Some sediment materials, particularly clay-based sediments, can be extremely xe2x80x9clossyxe2x80x9d. This lossy behavior of the soil is exhibited by a severe attenuation of an electromagnetic pulse as it propagates along a transmission line surrounded by such materials. The pulse, when launched from a TDR, dissipates as it travels along the transmission line. Sufficient dissipation reduces the reflected pulse energy below a detectable level. For lossy consolidated soils, such as clay, the electromagnetic signal is attenuated greatly as it propagates along transmission line leads embedded in these soils. Levels of signal attenuation may be as great as tens of dBs/m in clay, yielding undetectable reflected signals in some cases. To protect the transceiver from scour action in a stream, it may be beneficial to bury it in the sediment below the expected level of scour. In this scenario, the soil, typically clay, may absorb all or most of a pulse""s energy, some on transmission, and the rest on reflection. For the case in which the transceiver is located in the water above the sediment, a pulse will be minimally attenuated in the water and will reflect strongly from the boundary with the sediment, the sediment having a significantly different refractive index. This occurs because the amplitude of the reflection correlates directly to the ratio between the refractive indices at the boundary.
In either of the above scenarios, once a portion of the transmitted pulse is reflected from the water/sediment boundary, the remainder of the pulse propagates to the end of the transmission line leads whereupon it also is reflected. If the reflection from the water sediment/boundary is difficult to detect, the reflection of its complement that must traverse the entire distance of the transmission line will be even more difficult to detect. Discernment of the occurrence of these two significant events thus complicates the problem of identifying a location at which scour in a streambed is occurring, for example.
Thus, needed is a real time scour detection and monitoring system that uses information gleaned from its own operation to set optimal operating parameters for purposes of establishing reflected signals that are able to be differentiated. Further, this system should be both operationally and fiscally efficient, able to broadcast continuous data, if need be, in real time using inexpensive narrow-bandwidth transmitters and data processors.
A system for efficiently and cost effectively monitoring the status of the interface between two dissimilar media is provided. The system uses principles applied from the theory of time domain reflectometry (TDR), together with novel circuitry and low cost narrow band telemetry, to provide real time monitoring on a continuous basis, as needed.
In a preferred embodiment, a system employing TDR techniques using a pulsed signal generator but having novel circuitry unique to this invention, is emplaced in an environment that permits access to a boundary between one media and a second media of interest. This may be, e.g., a streambed in which the first media is water and the media of interest is the sediment thereunder. Using basic principles of TDR, an electromagnetic pulse is imposed on parallel transmission lines embedded so as to traverse portions of both media, traversal through the interface therebetween being of most importance. The time of travel of this pulse to a first boundary, that is ostensibly the boundary of interest, is used in a feedback line to establish the pulse repetition frequency of operation of the pulse generator of the system via operation of a portion of the circuitry that is unique to this invention. The reflected pulse is also provided to a signal processing circuit that prepares the pulse for transmission on a low cost narrowband telemetry system.